Thermal process for removing solvent from polybenzimidazole continuous filamentary materials

ABSTRACT

AN IMPROVEMENT IN THE PROCESS FOR THE REMOVAL OF SOLVENT FROM SOLVENT-CONTAINING FIBROUS SUCH AS DRY-SPUN POLYBENZIMIDAZOLE CONTINUOUS FILAMENTARY MATERIALIS DISCLOSED. THE AS-SPUN CONTINUOUS FILAMENTARY MATERIALIS HEATED TO SUBSTANTIALLY REMOVE THE SOLVENT CONTAINED THEREIN WITHOUT SUBSTANTIAL OXIDATIVE DEGRADATION OF THE MATERIAL.

United States Patent O ABSTRACT OF THE DISCLOSURE An improvement in the process for the removal of solvent from solvent-containing fibrous materials such as dry-spun polybenzimidazole continuous filamentary material is disclosed. The as-spun continuous filamentary material is heated to substantially remove the solvent contained therein without substantial oxidative degradation of the material.

BACKGROUND OF THE INVENTION The preparation of polybenzimidazole continuous filamentary materials (e.g., monofilaments and multifilament yarn, strand, cable, tow, etc.) is well known in the art. Generally, a polybenzimidazole is dissolved in a suitable solvent and the solution treated, as by spinning, to produce the continuous filamentary material. The as-spun continuous filamentary materials often contain substantial amounts of residual spinning solvent therein. Such high amounts of contained residual spinning solvent are generally detrimental to the physical properties of the material.

The solvent-containing as-spun material, for example, has relatively low tensile strength. Other physical properties such as initial modulus, etc., also are generally poor.

The presence of residual solvent is also detrimental to the successful performance of other steps often applied to as-spun materials. For example, drawing of the polybenzimidazole continuous filamentary materials is a well known method for increasing the tensile properties, and particularly tenacity, of the materials. It has been found, however, that polybenzimidazole continuous filamentary materials often explode during a drawing operation resulting in a porous product having relatively low tensile properties.

The explosion problem is believed due in large part to the presence of moisture such as residual spinning solvent in the polybenzimidazole continuous filamentary materials. Residual solvent will also plasticize the fiber during the drawing operation preventing adequate orientation and attainment of target properties. Much attention has therefore been directed to the removal of the solvent from the formed continuous filamentary material. Solvent removal has heretofore been accomplished by washing with a solvent-miscible liquid. The washing step, however, essentially replaces one liquid with another. The presence of any residual washing liquid is also detrimental in drawing. The washed continuous filamentary materials therefore have to be subjected to extensive treatments to remove the wash liquid prior to drawing. These treatments are difiicult and expensive. For example, US. Pat.

No. 3,584,104 discloses washing as-spun polybenzimidazole fiber until it contains less than one percent by weight of the solvent and preferably until the material is essentially free of the solvent. Water is typically used as the wash liquid although other wash materials such as acetone, methanol, methyl ethyl ketone and the like are disclosed to be useful. The washing operation can be conducted by winding the polybenzimidazole continuous filamentary material on perforated rollers or bobbins, immersing the wound bobbins in the liquid wash water 3,814,794 Patented June 4, 1974 ice and pressure washing the fiber for about 2 to 100 hours or more. Pressure washing is accomplished by passing the wash medium into the interior of the hollow, perforated bobbin and outwardly through the perforations so that the solvent is flushed from the polybenzimidazole material wound thereon. The washed material must then be dried to remove the wash liquid to avoid the explosion problem as noted above.

The solvent-wash procedure is an expensive and time consuming operation requiring time to wind the bobbins, wash time (2 to 100 hours or more) and also time to dry the washed material of the wash liquid.

It is an object of this invention to provide a process for treating as-spun polybenzimidazole continuous filamentary materials to substantially remove residual spinning solvent, which process does not require a washing step.

It is an object of this invention to provide a process for treating as-spun polybenzimidazole continuous filamentary materials without substantial oxidative degradation of the materials.

It is an object of this invention to enhance the physical properties of as-spun polybenzimidazole filamentary material at a relatively low cost and in a relatively short time.

It is also an object of this invention to provide a continuous method for treating as-spun polybenzimidazole continuous filamentary material to substantially remove residual spinning solvent.

SUMMARY OF THE INVENTION These and other objects of the invention are provided by a process for removing residual spinning solvent from a spun polybenzimidazole continuous filamentary material containing from about 5 to about 25 percent by weight of residual solvent comprising:

(a) continuously introducing a continuous length of said residual solvent-containing polybenzimidazole continuous filamentary material into a heating zone at a temperature of from about 200 to about 400 C.;

(b) continuously passing said continuous length of said polybenzimidazole continuous filamentary material under constant tension through said heating zone for a residence time of from about 5 to about minutes; and

(c) continuously withdrawing said resulting continuous length of polybenzimidazole continuous filamentary material from said heating zone having a substantially reduced residual solvent content.

DESCRIPTION OF PREFERRED EMBODIMENTS The starting polymer Polybenzimidazoles are a known class of heterocyclic polymers which consist essentially of recurring units of the following formulas I and II. Formula I is:

wherein R is a tetravalent aromatic nucleus, with the nitrogen atoms forming the benzimidazole rings being paired upon adjacent carbon atoms, i.e., ortho carbon atoms, of the aromatic nucleus, and R is a member of the class consisting of an aliphatic (alkylene) group, a cycloaliphatic ring, an aromatic ring and a heterocyclic ring such as pyridine, pyrazine, furan, quinoline, thiophene, and pyran.

Formula II is:

wherein Z is an aromatic nucleus having the nitrogen atoms forming the benzimidazole ring paired upon adjacent carbon atoms of the aromatic nucleus.

Preferably, the continuous filamentary materials are prepared from aromatic polybenzimidazoles, that is, from polymers consisting essentially of the recurring units of Formula II and of Formula I wherein R' is an aromatic ring or a heterocyclic ring.

As set forth in the U.S. Pat. 3,174,947 and Reissue Pat. 26,065, which are incorporated herein by reference, the aromatic polybenzimidazoles having the recurring units of Formula II may be prepared by self-condensing a trifunctional aromatic compound containing only a single set of ortho disposed diamino substituents and an aromatic, preferably phenyl, carboxylate ester substituent. Exemplary of polymers of this type is poly-2, (6)-benzimidazole prepared by the auto-condensation of phenyl-3,4-diamino' benzoate.

As also set forth in the above-mentioned patents, the aromatic polybenzirnidazoles having the recurring units of Formula I may be prepared by condensing an aromatic tetraamine compound containing a pair of orthodiamino substituents on the aromatic nucleus with a dicarboxyl compound selected from the class consisting of (a) the diphenyl ester of an aromatic dicarboxylic acid, (b) the diphenyl ester of a heterocyclic dicarboxylic acid wherein the carboxyl groups are substituents upon carbon in a ring compound selected from the class consisting of pyridine, pyrazine, furan, quinoline, thiophene and pyran and (c) an anhydride of an aromatic dicarboxylic acid.

Examples of aromatic polybenzimidazoles which have the recurring structure of Formula I and which may be formed into fibers or yarns include:

poly-2,2'- (m-phenylene) -5 ,5 '-bibenzimidazole; poly-2,2'-(pyridylene-3",5 -5 ,5 '-bibenzimidazole poly-2,2'-(furylene-2,5 -5 ,5 -bibenzimidazole poly-2,2'-(naphthalene-1",6" -5 ,5 '-bibenzimidazole; poly-2,2'- (biphenylene-4",4"' -5 ,5 -bibenzimidazole; poly-2,2-amylene-5 ,5 -bibenzimidazole; poly-2,2'-octamethylene-5 ,5 -bibenzimidazole; poly-2,6- (m-phenylene -dimidazob enzene; poly-2,2'-cyclohexenyl-5 ,5 -bibenzimidazole; poly-2,2- m-phenylene -5 ,5 -di (benzimidazole) ether; poly-2,2'- m-phenylene -5 ,5 '-di (benzimidazole) sulfide; poly-2,2'-(m-phenylene -5 ,5 '-di (benzimidazole) sulfone; poly-2,2-(m-phenylene) -5 ,5 '-di (benzimidazole) methane; poly-2,2- (m-phenylene -5 ',5"-di(benzimidazole) propane-2,2; and poly-2',2"- (m-phenylene) -5 ,5-di(benzimidazole) ethylene- 1,2

where the double bonds of the ethylene groups are intact in the final polymer.

The preferred aromatic polybenzimidazole fiber or yarn is one prepared from poly-2,2-(m-phenylene)-5 ,5 '-bibenzimidazole, the recurring unit of which is:

Any polymerization process known to those skilled in the art may be employed to prepare the polybenzimidazole which may then be formed into a continuous filamentary material and subjected to the drying treatment of the present invention. Preferred techniques used to prepare the polybenzimidazole are disclosed in U.S. Pats. 3,509,108 and 3,551,389, both assigned to the assignee of the present invention and both herein incorporated by reference.

With respect to aromatic polybenzimidazoles, preferably equimolar quantities of the monomeric tetraamine and dicarboxyl compound are introduced into a first stage melt polymerization reaction zone and heated therein at a temperature above about 200 0., preferably at least 250 C., and more preferably from about 270 to 300 C. The reaction is conducted in a substantially oxygen-free atmosphere, i.e., below about 20 p.p.m. oxygen and preferably below about 8 p.p.m. oxygen, until a foamed prepolymer is formed. Usually, the first stage reaction is continued until a prepolymer is formed having an inherent viscosity, expressed as deciliters per gram, of at least 0.1, and preferably from about 0.13 to 0.3, the inherent viscosity (I.V.) as used in the present specification and claims being determined from a solution of 0.4 grams of the polymer in 100 ml. of 97 percent H at 25 C.

After the conclusion of the first stage reaction, which normally takes at least 0.5 hours, and preferably 1 to 3 hours, the foamed prepolymer is cooled and then powdered or pulverized in any convenient manner. The resulting prepolymer powder is then introduced into a second stage polymerization reaction zone wherein it is heated under substantially oxygen-free conditions, as described above, to yield a polybenzimidazole polymer product, desirably having an I.V., as measured above, of at least 0.6, e.g., 0.80 to 1.1 or more.

The temperature employed in the second stage is at least 250 0., preferably at least 325 C., and more pref erably from about 350 to 425 C. The second stage reaction generally takes at least 0.5 hours, and preferably from about 1 to 4 hours or more.

A preferred method of preparing the polybenzimidazole is disclosed in the aforesaid U.S. Pat. 3,509,108. As disclosed therein, aromatic polybenzimidazoles may be prepared by initially reacting the monomer in a melt phase polymerization at a temperature above about 200 C. and a pressure above 50 p.s.i. (e.g., 300 to 600 p.s.i.) and then heating the resulting reaction product in a solid state polymerization at a temperature above about 300 C. (e.g., 350 to 500 C.) to yield the final product.

Preparation of the continuous filamentary material The term continuous filamentary material as used herein is intended to include monofilaments and multifilaments such as strand, yarn, cable, tow, and the like.

As is well known, the polybenzimidazoles are generally formed into continuous filamentary materials by solution spinning, that is, by dry or wet spinning a solution of the polymer in an appropriate solvent such as dimethylacetamide, dimethylformamide, dimethylsulfoxide or sulfuric acid (used only in wet spinning) through an opening of predetermined shape into an evaporative atmosphere for the solvent in which most of the solvent is evaporated (dry) or into a coagulation bath (wet), resulting in the polymer having the desired shape.

The polymer solutions may be prepared, for example, by dissolving sufiicient polybenzimidazole in the solvent to yield a final solution suitable for extrusion containing from about 10 to 45 percent by weight of polymer, based on the total weight of the solution, preferably from about 20 to 30 percent by weight.

One suitable means for dissolving the polymer in the solvent is by mixing the materials at a temperature above the atmospheric boiling point of the solvent, for example, 25 to 120 C. above such boiling point, and at a pressure of 2 to 1 atmospheres for a period of 1 to 5 hours.

Preferably, the above polymer solutions, after suitable filtration to remove any undissolved portions, are dry spun. For example, the solutions may be extruded through a spinneret into a conventional type downdraft spinning column containing a circulating inert gas such as nitrogen, noble gases, combustion gases or super heated steam. Conveniently, the spinneret face is at a temperature of from about 100 to 170 C., the top of the column from about to 220 C., the middle of the column from about to 250 C.,

and the bottom of the column from about 160 to 320 C. After leaving the spinning column, the continuous filamentary materials are taken up, for example, at a speed in the range of from about 50 to 350 meters per minute. Further details for a method of dry-spinning polybenzimidazole continuous filamentary materials are shown in US. Pat. 3,502,756, Bohrer et al., assigned to the same assignee as the present application, and this patent is hereby incorporated by reference.

The above polymer solutions may also be wet-spun. For example, the polybenzimidazole solutions may be extruded into a coagulation bath containing dimethylacetamide.

Other suitable wet-spinning coagulation baths include polyhydroxy aliphatic alcohols having 2 to 3 hydroxy groups and 2 to 6 carbon atoms as disclosed in US. Pat. 3,619,453 to Riggs, assigned to the same assignee as the present invention and herein incorporated by reference. Polybenzimidazoles formed by wet-spinning into coagulation baths containing sulfuric acid are not suitable for use in the resent invention.

The as-spun polybenzimidazole continuous filamentary materials generally contain from about 5 to about 25 percent by weight of the materials of residual spinning solvent. The solvent or other moisture content of these asspun materials should be substantiall removed to enhance the properties of the as-spun materials. It has been found, for example, that reduction of residual moisture content to a level below about 2.0, preferably below about 0.5, and most preferably below about 0.1, percent by weight of the materials, allows the materials to be drawn without a substantial explosion problem.

A continuous length of the as-spun polybenzimidazole continuous filamentary material may be continuously introduced into an appropriate heating zone, continuously passed therethrough under constant tension and continuously withdrawn therefrom with a substantially reduced residual spinning solvent content. The brief passage through the heating zone results in no substantial degradation of the fibrous material.

The polybenzimidazole continuous filamentary materials may be heated in the heating zone by any suitable heating means. For example, the materials may be continuously passed into a muffle or radiant heat furnace of appropriate length. Also, the continuous filamentary material may be continuously passed into contact with a heated contact surface such as a hot roll, shoe, pin or plate. Combinations of heating means may also be employed in the heating zone. That is, the continuous filamentary material may be continuously passed through a mufile furnace into contact with a hot shoe or vice versa.

As known in the art, polybenzimidazoles oxidize at high temperatures and turn dark. A loss in physical properties may also result from oxidation. It is therefore preferable that the polybenzimidazole continuous filamentary materials be heated in an inert atmosphere to prevent oxidation of the fibers. An air atmosphere may be used. The material, however, may darken slightly when air is used as the heating zone atmosphere.

Steam may also be used as the heating zone inert atmosphere. The polybenzimidazole continuous filamentary material may pick up water during handling and removal of this water during treatment in the heating zone provides a suitable inert atmosphere.

The continuous length of the polybenzimidazole continuous filamentary material is continuously passed through the heating zone at a temperature of from about 200 to about 400, preferably from about 300 to about 350 C., for a residence time of from about 5 to about 90, preferably from about 10 to about 30, minutes to substantially remove the spinning solvent therein without substantial oxidative degradation of the materials.

The polybenzimidazole continuous filamentary material is continuously passed through the heating zone at an essentially constant length. That is, as known in the art, the material is maintained under a slight tension as it passes through the heating zone. Polybenzimidazole continuous filamentary materials passed into a heating zone as above without being held under sufiicient tension to maintain a constant length of material could shrink up to about 13 percent and thus change the properties of the dried material. While an essentially constant length of material is most preferred, a slight amount of shrinkage is more preferable than any amount of stretching.

The dried polybenzimidazole continuous filamentary materials may then be drawn. Since polybenzimidazole materials have a high natural moisture regain level, the dried continuous filamentary materials should be main tained in a dry, preferably inert atmosphere until drawing is completed. The materials can be maintained, for example, in an atmosphere such as air, argon, nitrogen, carbon dioxide, steam, or the like. The atmosphere is also preferably non-oxidizing.

Drawing of the polybenzimidazole continuous filamentary materials may be performed in any suitable manner according to drawing procedures known to those skilled in the art. Particularly preferred drawing techniques are disclosed in US. Pat. 3,622,660 of George F. Ecker and Thomas C. Bohrer and US. Pat. 3,541,199 of Thomas C. Bohrer and Arnold J. Rosenthal. Both the patents are assigned to the same assignee as the present invention and are herein incorporated by reference. For example, the polybenzimidazole continuous filamentary materials may be drawn at high drawing speeds, that is, the speed of the supply roll may be on the order of 10 to 50 meters per minute or higher. The limiting factor of the drawing speed is the particular design of the drawing apparatus and the elastic properties of the fiber.

The polybenzimidazole continuous filamentary materials may be subjected to a hot drawing treatment in any convenient hot drawing apparatus or zone, for example, by passing the material over a heated surface such as a hot roll, shoe, pin or plate or by passing the material through a radiantly heated zone, such as a mufiie furnace.

Preferably, the continuous filamentary material is passed immediately from the drying step into the hot drawing zone in order to minimize exposure of the material to the atmosphere.

Hot drawing is suitably conducted at a temperature of above about 350 C. up to about the degradation temperature of the polybenzimidazole continuous filamentary material, preferably from about 375 to 650 C., and more preferably from about 390 to 480 C.

The polybenzimidazole continuous filamentary material may be drawn at any desired draw ratio below that at which they break, typically from above about 1.5 :1 to 4.5 :1, and preferably from about 2.5:1 to 3.5:1. The hot drawing residence time, i.e., the time during which the material is being heated, is usually from about 0.2 to 50 seconds, preferably 0.5 to 10 seconds, and more preferably 0.5 to 5 seconds. Substantially longer residence times at the elevated drawing temperatures may cause degradation. Other suitable polybenzimidazole drawing techniques will be apparent to those skilled in the art.

The resulting drawn polybenzimidazole continuous filamentary materials of the present invention are characterized by a high degree of thermal stability and show great resistance to degradation by heat, hydrolytic media and oxidizing media. They may be used, for example, in deceleration chutes for aircraft, chutes for re-entry capsules, high temperature dust collector bags, non-flammable clothing and fabrics and in spacesuits and flight clothing.

The invention is additionally illustrated in connection with the following examples which are to be considered illustrative of the present invention. It should be understood, however, that the invention is not limited to the specific details of the examples.

. 7 EXAMPLE I I A polybenzimidazole yarn, namely, poly-2,2-(m-phenylene)-5,5'ebibenzimidazole, is selected as the exemplary polybenzimidazole for use in carrying out the process of v 8 an elongation of 75.7 percent'and percent residual solvent.

The data in Tables I and II above indicate that the spinning solvent used for forming polybenz'nnidazole contains less than 0.05

this invention. 5 continuous filamentary material may be removed by the The Polymer is formed into an aHPun yam in the dlrect application ofheat without significant oxidative manner desribed in the Example I of US. Pa 3502 756 degradation of the materials. The samples of yarns treated to Bohrer et al. More particularly, a dope of dimethylby the prqcess of the Present Invention and parfiqularly acetamide containing 23 percent by weight of the polyas shown.m Examp 16 have .physlcal propirtles mer is extruded through a llO-hole jet into a dry spinning 1O parable.wlth the Sample treated m the nt l chamber containing superheated steam as the evaporative P winch takes many t i Process of atmosphere A 300/110 yam (110 filaments making up invention thus olfers substantial savings ln time and equi a yarn having an overall denier of 300) is formed from EXAMPLE the as-spun yarn. The yarn contains about 15 percent by weight of the material of residual dimethylacetamide. t. t, i

Samples of the 300/110 as-spun yarn are strung up to Yam S-amp 16 F of Examp 16 H i i troduced into the furnace of Example II maintained at a pass over two hot shoes inches contact length each) temperature of C and continue 1 d th and through a series of muffie furnaces at a speed of one th h f d us y Pass? meter per minute. The temperature of the shoes is mainrot-1g or a Farm of 30 minutes The resulting Y mined at C while the furnace tem eramre is varied contlnuously wlthdrawn from the furnace, has a residual o P solvent content of 0.2 percent by weight.

between 225 and 300 C. Also, the total muflle furnace The thusdried am is drawn in the same mann r th length is varied from 20 feet to 36 feet. Samples of the d y e e thus-heated yarn are measured to determine the tensile med yam of Examp 1e The.drawn ls substamlauy strength, percent elongation and the amount of residual neg-gowns i gZ P h solvent. None of the heated samples show evidence of any e Presen i 1m} su tantla y removes t 6 substantial oxidative degradatiom nlng solvent contalned in the as-spun yarn and enhaces Details of the runs made and results obtained are shown the phxslcal Propertles tenslle Strength and Percent below in Table I. The data shows that substantial amounts elongatlon) of the P e P e heated of residual spinning solvent may be reduced with the procy the Process of the P esent mventlon has physlcal P P ess of the present invention. erties comparable with fiber samples treated with con TABLE I Muflle Shoe furnace Mufile Time of heating, Tensile temperatemperafurnace minutes strength, Percent Percent ture, ture, length, grams] elongaresidual Sample C. C. feet Shoe Furnace denier tion solvent 275 250 20 0. 6. 5 1. 59 61. 2 4. 5 275 300 20 0. 6 6. 5 2. 04 66. 6 3. o 275 225 as 0. 6 11. 5 1. 44 70. s 7. 1 275 250 as 0. 6 11. 5 1. 45 79. 0 6. 5

EXAMPLE II ventional pressure-washing techniques for substantially longer times.

Samples of the as-spun yarn of Example I are dried by The principles, preferred embodiments and modes of continuously introducing the yarn into a 36-foot furnace operation of the present invention have been described having a dry, flowing nitrogen atmosphere and continuous- 45 in the foregoing specification. The invention which is iny passing the y therethrough at a Speed of one meter tended to be protected herein, however, is not to be eon-v per minute at varying furnace temperatures. The construed as limited to the particular forms disclosed, since tinuously withdrawn yarn has a substantially reduced these are to be regarded as illustrative rather than restricresidual solvent content. Measurements similar to those tive. Variations and changes may be made by those skilled of Example I are made of the resulting yarns. The results in the art without departing from the spirit of the invenare shown below in Table II. None of the samples appear tion. to have any substantial oxidative degradation. I claim:

Yarn samples F, G and H are drawn without a substan- 1. An improved process for producing drawn poly. tial explosion problem utilizing the techniques shown in benzimidazole continuous filamentary materials which the Example of the aforesaid U.S. Pat. 3,541,199. That is, comprises: I the am is drawn by passing the yarn over a hot shoe (a) wet or dry solution spinning a polybenzimidazole y I o a l at a draw ratio of 2.0:1 at a drawing supply speed of to yield an as-spun polybenzlmldazole continuous 50 meters per minute and a temperature of 420 C. The filamentary, material containingfrom about 5 to resulting yarn is substantially non-porous and has good about 25 percent by weight of residual spinning solphysical properties. vent;

TABLE II Tensile Furnace Furnace Time of strength, Percent Percent temperlength, heating, grams] elonresidual Sample ature,C. feet minutes denier gatlon solvent 275 36 11.5 1. 66 55. 9 4. 3 s00 36 11.5 1.89 79.8 1. 6 325 3c 11. 5 1. 77 56. o 1. 15 350 36 11. 5 1. 69 43. 7 0. 39

For comparative purposes, samples of the as-spun yarn 70 (b) continuously introducing, without any prewashing are wound on a perforated bobbin and washed by pressure to remove said spinning solvent, a continuous length washing in the conventional manner such as described in of a said as-spun, residual solvent-containing poly- U.S. Pat. 3,584,104 for about 48 hours. The washed yarn benzimidazole continuous filamentary materialinto is dried in a muflle oven at C. for minutes. The a heating zone at a temperature of from about 200 resulting yarn has a tensile strength of 1.4 grams/denier, 75 to about 400 C., passing said continuous length of.

said polybenzimidazole continuous filamentary material through said heating zone for a residence time of from about to about 90 minutes, said filamentary material being held under a tension sulficient to maintain an essentially constant length of material as it passes through said heating zone,

(c) continuously withdrawing said resulting continuous length of polybenzimidazole continuous filamentary material from said heating zone having a substantially reduced residual solvent content; and

(d) drawing the resulting filamentary material obtained from said heating zone in a drawing zone at a draw ratio of from about 1.5:1 to 45:1 and at a temperature above about 350 C. up to about the degradation temperature of the polybenzimidazole continuous filamentary material for about 0.2 to 50 seconds to yield a drawn polybenzimidazole continuous filamentary material.

2. The process of claim 1 wherein said polybenzimidazole continuous filamentary material withdrawn from said heating zone has a residual moisture content less than about 2 percent by weight.

3. The process of claim 2 wherein the said heating zone is at a temperature of from about 300 to about 350 C. and wherein the said continuous filamentary material has a residence time in the heating zone of from about to about 30 minutes.

4. The process of claim 3 wherein the spinning solvent is dimethylacetamide.

5. The process of claim 3 wherein the spinning solvent is dimethylformamide.

6. A process for enhancing the properties of a dry solution spun polybenzimidazole continuous filamentary material containing from about 5 to about 25 percent by weight of residual spinning solvent which comprises:

(a) continuously introducing, without any prewashing to remove said spinning solvent, a continuous length of said as-spun, residual solvent-containing polybenzimidazole continuous filamentary material into a heating zone at a temperature of from about 200 to about 400 C., passing said continuous length of said polybenzimidazole continuous filamentary material 10 through said heating zone for a residence time of from about 5 to about 90 minutes, said filamentary material being held under a tension suttficient to maintain an essentially constant length of material 5 as is passes through said heating zone,

(b) continuously withdrawing said resulting continuous length of polybenzimidazole continuous filamem tary material from said heating zone having a residual moisture content of less than about 2.0 percent by weight; and

10 (c) drawing the resulting filamentary material in a drawing zone at a temperature of from about 375 to 650 C. and at a draw ratio of from about 1.5 :l to 4.5 :1 for about 0.2 to 50 seconds to yield a drawn polybenzimidazole continuous filamentary material.

7. The process of claim 6 wherein the heating zone is at a temperature of from about 300 to about 350 C., wherein the residence time of the filamentary material in the heating zone is from about 10 to about 30 minutes,

and wherein the filamentary material withdrawn from said heating zone has a residual moisture content less than about 0.5 percent by weight.

References Cited UNITED STATES PATENTS 2,210,161 8/1940 Allen 264-205 2,433,842 1/ 19481, Griflin 28-62 2,692,875 10/ 1954 Weinstock et al. 28-62 30 2,811,409 10/1957 Clapp et al. 264-206 2,975,022 3/ 1961 Taylor 264-206 3,523,151 8/1970 Steinberg 264-210 F 3,526,693 9/1970 Rulison et al 264-210 F 3,584,104 6/1971 Bohrer et al. 264-210 F 3,619,453 11/1971' Riggs 264-210 F 3,622,660 11/1971 Ecker et al. 264-290 R 3,657,211 4/1972 Bohrer et al. 264-204 JAY H. WOO, Primary Examiner U.S. c1. X.R. 

